The natural history of group B streptococcal (GBS) infections is being investigated. Longitudinal, prospective clinical, epidemiological and serological studies are concentrated in the area of puerperal and perinatal infections. Maternal GBS carrier/infection rates are determined during pregnancy and at delivery. GBS acquisitions in prospectively studied newborns are defined according to culture sites, and the degree of exposure to GBS at or around time of birth is correlated with risk of disease. The spectrum of disease in infancy is defined and attack rates of early and late sepsis are determined. Maternal factors possibly predisposing to GBS infection are evaluated. The role of GBS in UTI in women with complicated pregnancies is being studied in separate but related investigation. Studies of host defense include assays for type-specific Ab in parturients and offspring and the relation between Ab and disease incidence. The role of homologous and heterologous carriage in the evolution of antibody is being explored. An ELISA method has been devised for sero-epidemiological study. Protective Ab assays include the chick embryo and mouse protection models and indirect bactericidal test for opsonic Ab. Long-term trends in maternal carriage/infection and neonatal disease caused by GBS will be determined in the course of this investigation in anticipation of the possibility for implementing and evaluating prophylactic or preventive measures.